Harris Corner Detection Theory of Shape Based Matching Application

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2.1.1.5 Harris Corner Detection

A corner can be defined as the intersection of two edges or point for which there are two dominant and different edge directions in a local neighborhood of the point. The Harris Corner Detector HCD also known as Harris Point is a popular interest point detector due to its strong invariance to Schmid et al., 2000: rotation, scale, illumination variation and image noise. The Harris corner detector is based on the local auto-correlation function of a signal; where the local auto-correlation function measures the local changes of the signal considering the differential of the corner score with respect to the direction directly. In this research, a fundamental of the corner point detection will be introduced. The Harris corner detection Harris and Stephens, 1988 method avoids the explicit computation of the eigenvalues of the sum of squared differences matrix by solving the corner metric. A corner is characterized by a large variation of M in all direction of the vector x, y. By analyzing the eigenvalues of M , this characterization can be expressed in the following way: 1. If both ʎ 1 , ʎ 2 are small, so that the local auto-correlation function is flat little changes in R any direction, the windowed image region is of approximately constant intensity. 2. If one eigenvalue is high and the other low, so the local auto-correlation function is ridge shaped, then only local shifts in one direction along the ridge cause little change in R and significant change in the orthogonal direction; this indicates an edge. 33 3. If both eigenvalues are high, so the local auto-correlation function is sharply peaked, then shifts in any direction will results in a significant increase; this indicates a corner. In this research, each localization point occurs need to be classified by using the 3 conditions as state before. This is important for applying only a corner characteristic that will be used for indicating a critical point of object. Figure 2.14 shows the relation between the object with corner response map in determining the characteristic of each points occurred. Edge, flat and corner is represent using a simple example of square object red square combined with their corresponding region and range of determinant value as shown in corner response graph in Figure 2.14. Figure 2.13: Harris Point Application Schmid et al., 2000 34

2.1.1.6 Template Matching